In the manufacture of integrated circuits and other electrical devices, lithographic processes are used in which a circuit pattern is exposed on a layer of photoresist formed on a substrate, utilizing radiation of various types. In accordance with techniques well-known in the art, such lithographic processes typically entail development of the exposed resist layer, followed by one or more etching processes in which the exposed areas are removed, whereby a circuit pattern may subsequently be replicated in metal or semiconductor layers beneath the exposed pattern.
Because of the necessity in modern circuit components for precise yet highly miniaturized circuit patterns, such lithographic techniques must produce printed patterns of very high resolution, and conventional photolithographic techniques have proven unsatisfactory in some applications because of inherent technical limitations. Conventional photolithographic techniques achieve sub-micron resolution only with difficulty because of the natural phenomenon of diffraction of light utilized in shadowgraph exposures, which tends to cause poor image quality and a small depth of focus.
Although electron beam and X-ray lithographic techniques exist which are capable of achieving sub-micron resolution, they involve certain limitations for some applications. For example, in electron beam lithography, when utilized in a high resolution, parallel-printing process, critical dimension control problems are experienced due to a proximity effect inherent in the electron beam process. X-ray lithography tends to be expensive and in general tends to require undesirably long exposure time to expose a resist adequately. X-ray lithography also involves diffraction effects which can limit resolution.
Ion beam lithography is a technique which is capable of producing precise, very high resolution patterns and which is not limited by proximity effect problems such as those of parallel printing electron beam lithography. Moreover, commercially available sources for generating highly collimated, high energy ion beams are substantially less expensive and more effective for exposing resists than even the brightest X-ray sources. However, when used in parallel-printing procedures, ion beams are easily blocked by materials which are considered transparent in the context of conventional photolithography or X-ray lithography. In fact, almost any solid material having a thickness greater than one micron will block the transmission of even high energy, e.g., 100 Kv, ion beams.
If the transmissive areas through which ions are passed are defined by very thin sections of material intended to provide semi-transparent "windows," i.e., sections thinner than one micron, the material tends to cause the ion beams to diverge, even in the case of highly collimated ion beams. In general, such masks are spaced from the surface upon which a pattern is being printed for preventing damage to the surface or the mask. Consequently, because of this spacing requirement and the divergence caused by such semi-transparent regions, highly precise patterns often are not obtainable.